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A fluorescence quenching sensor for Fe3+ detection using (C6H5NH3)2Pb3I8·2H2O hybrid perovskite
Inorganic Chemistry Communications 109 (2019) 107562
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Short communication
A fluorescence quenching sensor for Fe3+ detection using (C6H5NH3)2Pb3I8·2H2O hybrid perovskite
T
Meng-Ya Zhua, Le-Xi Zhanga, , Jing Yinb, Jing-Jing Chena, Li-Jian Biea, ⁎
⁎
a
School of Materials Science and Engineering, Tianjin Key Lab for Photoelectric Materials and Devices, Key Laboratory of Display Materials and Photoelectric Devices (Ministry of Education), Tianjin University of Technology, Tianjin 300384, China b School of Environmental Science and Safety Engineering, Tianjin University of Technology, Tianjin 300384, China
GRAPHICAL ABSTRACT
Organic-inorganic hybrid 1D perovskite (C6H5NH3)2Pb3I8·2H2O has been synthesized. Employed as a Fe3+ sensing material based on fluorescence quenching, it shows excellent performance for Fe3+ detection in DMF solution, with a LOD of 7.51 × 10−8 mol/L. The mechanism of fluorescence quenching can be attributed to Fe3+ inhibited radiative electron-hole recombination via capturing electrons.
ARTICLE INFO
ABSTRACT
Keywords: Hybrid perovskite (C6H5NH3)2Pb3I8·2H2O Fluorescence quenching sensor Ferric cations Radiative electron-hole recombination
A new organic-inorganic hybrid perovskite (C6H5NH3)2Pb3I8·2H2O single crystal has been synthesized through a facile solution method. In this perovskite, there exists a 1D infinite lead iodide chains constituted by Pb3I8 groups, which is surrounded by anilines. As an active fluorescence quenching sensor material, this perovskite shows excellent performance for Fe3+ detection in N, N-dimethylformamide (DMF) solution with a detection limit of 1.0 × 10−7 mol/L, including short response time, high sensitivity and high selectivity. The sensitivity and selectivity towards Fe3+ is much higher than that towards other metal cations, which provides a facile way for detecting Fe3+ cations in solution. Electron paramagnetic resonance (EPR) confirms that the mechanism of fluorescence quenching can be attributed to Fe3+ inhibition to the radiative electron-hole recombination via capturing electrons.
The past several years have witnessed potential advantages of organic-inorganic hybrid perovskites in optoelectronic applications [1–3]. In this field, especially for solar cells, instability caused by electron-hole recombination is one of the main problems that need to be addressed [4,5]. Researches mostly focused on decreasing electron-hole recombination in AMX3 and A2MX4 through introducing hole or electron-
⁎
transport materials [6,7]. As a matter of fact, organic-inorganic hybrid perovskites are interesting semiconductor materials [8]. For three-dimensional (3D) network AMX3, the charge carrier transports along corner-sharing MX62− octahedra [9]. The size of cation A is limited by the structure of 3D network [10], which affects the formability in different perovskites. However, the size of organic cation is no longer
Corresponding authors. E-mail addresses: [email protected] (L.-X. Zhang), [email protected], [email protected] (L.-J. Bie).
https://doi.org/10.1016/j.inoche.2019.107562 Received 15 July 2019; Received in revised form 14 August 2019; Accepted 31 August 2019 Available online 31 August 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 109 (2019) 107562
M.-Y. Zhu, et al.
Fig. 1. Crystal structure of (C6H5NH3)2Pb3I8·2H2O (a, b); bird view along c-axis (c) and infinite extension lead iodide chains (d).
confined in a two-dimensional (2D) A2MX4 layered perovskite [9]. The inorganic component provides the opportunity for higher carrier mobility, meanwhile organic component offers the possibility of structural diversity [11]. 2D materials form repeating multi-quantum well structures, the carriers are limited in the inorganic component layer formed by MX62− octahedron, and the organic part acts as a potential barrier, in which the particular structure characteristic leads to the existence of excitons with large binding energy [12]. Recently, attentions have been focused on 1D [13] and 0D [14] perovskites, due to the strong quantum confinement and site isolation. Among them, 1D perovskites have good potential for applications involving radiative hole-electron recombination due to their low exciton dissociation efficiency [13]. It is reported that ionic movement influences recombination in perovskite solar cells [4]. As fluorescence is an important way to release energy via radiative electron-hole recombination, view from the sensing point, external metal cations may have influences on radiative electron-hole recombination in 1D perovskites, so 1D perovskites might be good fluorescence sensors for detecting metal cations. As known to all, iron is one of the essential transition metals in living things, and it is of vital importance to detect iron cation in solution sensitively and selectively. Previously, methods reported for detecting iron include electrochemical methods [15], atomic absorption spectrometry (AAS) [16], inductively coupled plasma-atomic emission spectrometry (ICP-MS) [17], and so on. Despite the good sensitivity and selectivity, electrochemical method had drawbacks such as toxicity derived from mercury electrode and the longer chelating reaction time [15]. Spectrometric analysis required sophisticated apparatus and expensive cost [15,18]. It is still a challenge to develop a facile way or a novel material for fast iron detection with high sensitivity, selectivity. Recently, fluorescence quenching sensor to detect Fe3+ using such as Bi2S3-TiO2 [19] and metal organic frameworks [20,21] has been reported. In this work, a novel 1D organic-inorganic hybrid perovskite single crystal (C6H5NH3)2Pb3I8·2H2O synthesized through a solution method was reported, which exhibits excellent fluorescence quenching performance for Fe3+ cations, including short reaction time, low cost, high sensitivity and selectivity. And mechanism of the fluorescence quenching sensor was discussed via EPR as well from the inhibition of radiative electron-hole recombination.
All the chemical reagents used in the experiment were in analytical grade without further purification. Distilled water was employed throughout the experiment. In a typical synthesis, 6 ml aniline was dissolved in 30 ml HI (47 wt%) and 30 ml absolute C2H5OH solution in an ice-water bath. After magnetic stirring for 30 min, the mixed solution was maintained at 90 °C for 4 h in a water bath, resulting in the formation of white acicular crystals. The white acicular crystals were collected, washed several times with absolute C4H10O, and dried in a vacuum oven for 12 h at 70 °C to obtain C6H5NH3I. After that, 1.1574 g PbI2 and 1.1065 g C6H5NH3I were mixed in solution composed of 10 ml HI (47 wt%) and 10 ml absolute C2H5OH. Under strong magnetic stirring for 30 min, the solution was kept at 90 °C for 7 h in a water bath. Light yellow acicular single crystal (C6H5NH3)2Pb3I8·2H2O can be obtained after crystallization in two weeks, which was collected with vacuum filtered, washed several times with absolute ether, and dried in a vacuum oven for 12 h at 70 °C. The scanning electron microscope (SEM) image of the sample was examined using a JEOL JSM-6700F field-emission scanning electron microscope (FE-SEM), with an accelerating voltage of 10 kV. A suitable crystal with size of 0.220 × 0.210 × 0.180 mm was selected for single crystal X-ray diffraction analysis. Crystallographic data was collected on a Bruker Apex II CCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature. The structure was determined by the direct method using the SHELXTL-2014 program. To refine the structures, anisotropic thermal factors were employed for the non-H atoms. The hydrogen atoms of water in the crystal could not be found in the Fourier map because of the disordered arrangement. UV–Vis absorption spectra were recorded on a Shimadzu UV-2550 spectrometer. Fluorescence spectra were measured with a Hitachi F-4600 fluorescence spectrophotometer by a 280 nm excitation from a Xenon lamp as the excitation source at room temperature. The optical images were conducted on the fluorescent inverted microscope Nikon Eclipse TE2000U with 20 times magnification under UV lamp. Electron paramagnetic resonance (EPR) was carried out on Bruker EMX Plus at room temperature. The crystal structure of (C6H5NH3)2Pb3I8·2H2O was shown in Fig. 1, determined by single crystal X-ray diffraction. There are three Pb atom positions (Pb1, Pb2, Pb3) in one unit cell, with slightly distorted octahedral coordination. Each Pb atom is hexa-coordinated by iodine 2
Inorganic Chemistry Communications 109 (2019) 107562
M.-Y. Zhu, et al.
Fig. 2. (C6H5NH3)2Pb3I8·2H2O dispersed in DMF: (a) UV–Vis absorption spectra, inset is the absorbance v.s. the concentration ratio of Fe3+ over the sample, (b) fluorescence titration spectra, (c) the fluorescence intensity, inset is the corresponding calibration between the fluorescence intensity and Fe3+ concentration; Fluorescence results in blank and presence of various metal cations: (d) emission spectra, (e) fluorescence intensity, (f) selectivity.
Fig. S3. The sample shows bright fluorescence in Fe3+ blank (Fig. S3a), fluorescence quenching appears as the Fe3+ concentration is 1.0 × 10−5 mol/L, which is consistent with the result in Fig. 2(b). The fluorescence intensity at 339 nm was plotted as a function of the Fe3+ concentrations, as shown in Fig. 2(c), the inset illustrates Fe3+ concentration v.s. emission intensity, showing a linear relation. Quantitatively, the effect can be fitted as the equation If = 475.9–3.303 × 106 CFe(III), with a linear factor of R2 = 0.9965, meaning that Fe3+ has a strong interaction with the sample. The association constant (Ka) is 3.303 × 106, revealing an extremely strong quenching effect on the (C6H5NH3)2Pb3I8·2H2O sample [22,23]. The detection limit was measured to be as low as 1.0 × 10−7 mol/L through the titration method, which is superior to other methods regarding Fe3+ detection in solutions [24–26]. In addition, all the fluorescence data were recorded directly in the Fe3+ titration process, implying fast response, good for rapid detecting Fe3+ quantitatively. Based on the equation LOD = 3δ/s (where δ is the standard deviation of the signals and s is the slope of the linear calibration plot) [26,27], the limit of detection (LOD) was calculated to be 7.51 × 10−8 mol/L. The linear range and LOD of fluorescence sensing materials for Fe3+ detection were summarized in Table 1 [26–31]. The linear range and LOD of the developed sensing material (C6H5NH3)2Pb3I8·2H2O are improved over those of most reported fluorescence sensing materials in Table 1. It is worth noting that the
atoms. Carbon atoms in aniline vibrate at two symmetric positions, and water molecules situated between layers (Fig. 1a and b). Obviously, it is a 1D structure isolated by larger organic aniline. The conduction band (CB) and valence band (VB) of these perovskites are derived primarily from the inorganic lead iodine octahedron layer. Viewing along c-axis, layers are composed of alternate organic aniline groups and inorganic octahedra (Fig. 1c), which is favorable for electron-hole recombination between VB and CB. Infinite lead iodide octahedral chains are constituted by the Pb3I8 groups, surrounded by anilines (Fig. 1d), which might be a good structural characteristic for a fluorescence quenching sensor. The morphology of the obtained crystal is acicular, with a length of ~2 mm, width of ~30 μm, and thickness of ~8 μm (Fig. S2), consistent with the self-regulation principle in crystal growth. Crystallographic data has been deposited as CCDC 1864099, and crystal details and refinement results for (C6H5NH3)2Pb3I8·2H2O can also be found in the Supporting Information. Fluorescence quenching performance of (C6H5NH3)2Pb3I8·2H2O for detecting metal cation were investigated using a signal transduction pathway by titration in DMF. Since the as-synthesized sample is stable under sunlight, it was dispersed directly in DMF to form solution. Fig. 2 (a) illustrated the UV–Vis absorption spectra of the solution with different Fe3+ concentrations at room temperature. According to the UV–Vis absorption intensity of the solution, optimum excitation wavelength for the fluorescence measurement was chosen as 280 nm. At the excitation wavelength of 280 nm and a slit of 2.5 nm, the sample exhibits a broad fluorescence peak centered at 339 nm, as can be seen in Fig. 2(b). Fluorescence titration spectra were carried out in the presence of different Fe3+ concentrations ranging from 1.0 × 10−7 to 1.0 × 10−3 mol/L. The fluorescence intensity (If) decreases as the Fe3+ concentration (CFe(III)) increase from 1.0 × 10−7 to 1.0 × 10−3 mol/L, and the fluorescence is almost quenched as the Fe3+ concentration is 1.0 × 10−5 mol/L. In order to observe the fluorescence quenching effect intuitively, optical images of sample in DMF with Fe3+ concentrations ranging from 0 to 1.0 × 10−5 mol/L were recorded on a fluorescent inverted microscope with 20 times magnification under UV lamp, as presented in
Table 1 Comparison of fluorescence sensing materials for Fe3+ detection.
3
Sensing materials
Linear range (μmol·L−1)
LOD (μmol·L−1)
Ref.
FNCD N-CQDs [Zn2(oba)2(bpy)] Cd-MOF CuNCs AuNCs-PbS-QDs (C6H5NH3)2Pb3I8·2H2O
2–25 3.32–32.26 50–250 0–16 1–100 3–40 0.1–100
0.9 0.7462 0.3 0.3 0.3 1.5 0.0751
26 27 28 29 30 31 This work
Inorganic Chemistry Communications 109 (2019) 107562
M.-Y. Zhu, et al.
Fig. 3. EPR spectra of Fe3+ as absence and presence of (C6H5NH3)2Pb3I8·2H2O in DMF with the X-axis using g-factor (a) and magnetic field (b). Table 2 The standard electrode potential of different metal ions. Ion pairs
Mn2+/Mn
Ni2+/Ni
Cd2+/Cd
Ca2+/Ca
Mg2+/Mg
Zn2+/Zn
Cr3+/Cr
Co2+/Co
Al3+/Al
Pb2+/Pb
Fe2+/Fe
Hg2+/Hg
Fe3+/ Fe2+
E°/V
−1.030
−0.257
−0.403
−2.868
−2.372
−0.762
−0.744
−0.280
−1.662
−0.126
−0.447
0.790
0.770
lowest detection concentration for Fe3+ in this work is 0.1 μmol·L−1, which is even lower than the LOD of most reported materials [26–31]. Thinking about the interference of other metal cations, selectivity towards different metal cations was measured under same condition. As can be seen in Fig. 2(c), the fluorescence intensity was decreased by 97.41% with 4.0 × 10−4 mol/L Fe3+, so this concentration was chosen to investigate the selectivity of (C6H5NH3)2Pb3I8·2H2O with other 13 metal cations. The fluorescence emission spectra towards various metal cations are shown in Fig. 2(d). Influences on the fluorescence intensity at 339 nm are different with various metal cations, as illustrated in Fig. 2(e). Compared with other metal cations, Fe3+ displays obvious fluorescence quenching effect. The selectivity of sample to a certain metal cation (M) is defined as:
SM = (F0
hole recombination in the sample via capturing electrons. Standard electrode potential (E°) usually represents the ability of gaining electrons, the higher the electrode potentials, the higher the ability for getting electrons [40]. As listed in Table 2, standard electrode potential of Fe3+/Fe2+ and Hg2+/Hg+ cation pairs are high, so they exhibit stronger fluorescence quenching effect than other cations. Furthermore, ion diameter (D) might be another factor affecting the fluorescence quenching effect. The diameter of Fe3+ (D = 1.1 Ǻ) is smaller than that of Hg2+ (D = 2.0 Ǻ), thus fluorescence quenching effect of Fe3+ is higher than that of Hg2+ cation [40,41], as more charge with smaller ion diameter means stronger polarization and electron gaining potential. The CB and VB of these perovskites semiconductor are derived primarily from the inorganic lead iodine octahedra layer, so a proper ion size is needed to penetrate into the anionic chains for efficient electron transfer. The fluorescence quenching mechanism might be thought as the organic aniline layer inhibits metal ions with bigger diameter from approaching inorganic layer of lead iodine octahedra in the perovskite. On the basis of electron capture, Fe3+ shows the best fluorescence quenching effect via inhibition of radiative electron-hole recombination in the perovskite, which provides an effective method to detect Fe3+ quantitatively. In summary, a novel organic-inorganic hybrid perovskite (C6H5NH3)2Pb3I8·2H2O single crystal has been synthesized through a solution method. It has a unique perovskite structure that 1D lead iodide octahedron chains is constituted by the Pb3I8 groups, which is surrounded by anilines. Viewing from c-axis, layers are composed of alternate organic aniline groups and inorganic octahedra, which is favorable for radiative electron-hole recombination between the VB and the CB. This structure characteristic is good for fluorescence quenching sensor. The obtained organic-inorganic hybrid 1D perovskite (C6H5NH3)2Pb3I8·2H2O shows excellent fluorescence quenching performance as Fe3+ titration in DMF with a detection limit of 1.0 × 10−7 mol/L, including short reaction time, high sensitivity and selectivity. Based on the EPR results, the fluorescence quenching mechanism of (C6H5NH3)2Pb3I8·2H2O by Fe3+ can be attributed to the inhibition of radiative electron-hole recombination via capturing electrons. This work demonstrates the potential of these compounds in quantitatively detecting oxidizing metal cations.
F)/F0 × 100%
where F and F0 are the fluorescence intensity in metal cations presence and blank. Detailed selectivity of (C6H5NH3)2Pb3I8·2H2O towards other metal ions are shown in Fig. 2(f). The selectivity towards Fe3+ is much higher than other metal cations, with a SFe(III) value of 97.41%, showing very good selectivity [22,32,33]. Among the tested metal cations, Fe3+ exhibits obvious quenching effect on the sample. Since Fe3+ has five unpaired electrons in d orbital, electron paramagnetic resonance (EPR) was taken to investigate the fluorescence quenching mechanism. As shown in Fig. 3, the EPR spectra of Fe3+ are measured in blank and presence of (C6H5NH3)2Pb3I8·2H2O in DMF. The EPR signal at g = 2.16 in Fig. 3(a) can be ascribed to Fe3+ cation [34,35], and the signal observed in Fig. 3(b) can also be attributed to Fe3+ [36–38]. Furthermore, the concentration of the sample can be calculated qualitatively by integrating areas of detected peaks [35,39]. Comparing the EPR signal intensity of Fe3+, the intensity decreases obviously as sample presents. This implies that Fe3+ might react with (C6H5NH3)2Pb3I8·2H2O, leading to decreased Fe3+ concentration. Similar with light emission and absorption in MAPbI3, the electron-hole separation in (C6H5NH3)2Pb3I8·2H2O can generate photocurrent under sunlight radiation (Fig. S5), and the fluorescence is resulted from the radiative recombination of electrons and holes [5,8,12]. It is reasonable that Fe3+, as an oxidant, gain electrons from the sample, resulting in fluorescence quenching. Namely, Fe3+ inhibits the radiative electron4
Inorganic Chemistry Communications 109 (2019) 107562
M.-Y. Zhu, et al.
Acknowledgments
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A fluorescence quenching sensor for Fe3+ detection using (C6H5NH3)2Pb3I8·2H2O hybrid perovskite
T
Meng-Ya Zhua, Le-Xi Zhanga, , Jing Yinb, Jing-Jing Chena, Li-Jian Biea, ⁎
⁎
a
School of Materials Science and Engineering, Tianjin Key Lab for Photoelectric Materials and Devices, Key Laboratory of Display Materials and Photoelectric Devices (Ministry of Education), Tianjin University of Technology, Tianjin 300384, China b School of Environmental Science and Safety Engineering, Tianjin University of Technology, Tianjin 300384, China
GRAPHICAL ABSTRACT
Organic-inorganic hybrid 1D perovskite (C6H5NH3)2Pb3I8·2H2O has been synthesized. Employed as a Fe3+ sensing material based on fluorescence quenching, it shows excellent performance for Fe3+ detection in DMF solution, with a LOD of 7.51 × 10−8 mol/L. The mechanism of fluorescence quenching can be attributed to Fe3+ inhibited radiative electron-hole recombination via capturing electrons.
ARTICLE INFO
ABSTRACT
Keywords: Hybrid perovskite (C6H5NH3)2Pb3I8·2H2O Fluorescence quenching sensor Ferric cations Radiative electron-hole recombination
A new organic-inorganic hybrid perovskite (C6H5NH3)2Pb3I8·2H2O single crystal has been synthesized through a facile solution method. In this perovskite, there exists a 1D infinite lead iodide chains constituted by Pb3I8 groups, which is surrounded by anilines. As an active fluorescence quenching sensor material, this perovskite shows excellent performance for Fe3+ detection in N, N-dimethylformamide (DMF) solution with a detection limit of 1.0 × 10−7 mol/L, including short response time, high sensitivity and high selectivity. The sensitivity and selectivity towards Fe3+ is much higher than that towards other metal cations, which provides a facile way for detecting Fe3+ cations in solution. Electron paramagnetic resonance (EPR) confirms that the mechanism of fluorescence quenching can be attributed to Fe3+ inhibition to the radiative electron-hole recombination via capturing electrons.
The past several years have witnessed potential advantages of organic-inorganic hybrid perovskites in optoelectronic applications [1–3]. In this field, especially for solar cells, instability caused by electron-hole recombination is one of the main problems that need to be addressed [4,5]. Researches mostly focused on decreasing electron-hole recombination in AMX3 and A2MX4 through introducing hole or electron-
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transport materials [6,7]. As a matter of fact, organic-inorganic hybrid perovskites are interesting semiconductor materials [8]. For three-dimensional (3D) network AMX3, the charge carrier transports along corner-sharing MX62− octahedra [9]. The size of cation A is limited by the structure of 3D network [10], which affects the formability in different perovskites. However, the size of organic cation is no longer
Corresponding authors. E-mail addresses: [email protected] (L.-X. Zhang), [email protected], [email protected] (L.-J. Bie).
https://doi.org/10.1016/j.inoche.2019.107562 Received 15 July 2019; Received in revised form 14 August 2019; Accepted 31 August 2019 Available online 31 August 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 109 (2019) 107562
M.-Y. Zhu, et al.
Fig. 1. Crystal structure of (C6H5NH3)2Pb3I8·2H2O (a, b); bird view along c-axis (c) and infinite extension lead iodide chains (d).
confined in a two-dimensional (2D) A2MX4 layered perovskite [9]. The inorganic component provides the opportunity for higher carrier mobility, meanwhile organic component offers the possibility of structural diversity [11]. 2D materials form repeating multi-quantum well structures, the carriers are limited in the inorganic component layer formed by MX62− octahedron, and the organic part acts as a potential barrier, in which the particular structure characteristic leads to the existence of excitons with large binding energy [12]. Recently, attentions have been focused on 1D [13] and 0D [14] perovskites, due to the strong quantum confinement and site isolation. Among them, 1D perovskites have good potential for applications involving radiative hole-electron recombination due to their low exciton dissociation efficiency [13]. It is reported that ionic movement influences recombination in perovskite solar cells [4]. As fluorescence is an important way to release energy via radiative electron-hole recombination, view from the sensing point, external metal cations may have influences on radiative electron-hole recombination in 1D perovskites, so 1D perovskites might be good fluorescence sensors for detecting metal cations. As known to all, iron is one of the essential transition metals in living things, and it is of vital importance to detect iron cation in solution sensitively and selectively. Previously, methods reported for detecting iron include electrochemical methods [15], atomic absorption spectrometry (AAS) [16], inductively coupled plasma-atomic emission spectrometry (ICP-MS) [17], and so on. Despite the good sensitivity and selectivity, electrochemical method had drawbacks such as toxicity derived from mercury electrode and the longer chelating reaction time [15]. Spectrometric analysis required sophisticated apparatus and expensive cost [15,18]. It is still a challenge to develop a facile way or a novel material for fast iron detection with high sensitivity, selectivity. Recently, fluorescence quenching sensor to detect Fe3+ using such as Bi2S3-TiO2 [19] and metal organic frameworks [20,21] has been reported. In this work, a novel 1D organic-inorganic hybrid perovskite single crystal (C6H5NH3)2Pb3I8·2H2O synthesized through a solution method was reported, which exhibits excellent fluorescence quenching performance for Fe3+ cations, including short reaction time, low cost, high sensitivity and selectivity. And mechanism of the fluorescence quenching sensor was discussed via EPR as well from the inhibition of radiative electron-hole recombination.
All the chemical reagents used in the experiment were in analytical grade without further purification. Distilled water was employed throughout the experiment. In a typical synthesis, 6 ml aniline was dissolved in 30 ml HI (47 wt%) and 30 ml absolute C2H5OH solution in an ice-water bath. After magnetic stirring for 30 min, the mixed solution was maintained at 90 °C for 4 h in a water bath, resulting in the formation of white acicular crystals. The white acicular crystals were collected, washed several times with absolute C4H10O, and dried in a vacuum oven for 12 h at 70 °C to obtain C6H5NH3I. After that, 1.1574 g PbI2 and 1.1065 g C6H5NH3I were mixed in solution composed of 10 ml HI (47 wt%) and 10 ml absolute C2H5OH. Under strong magnetic stirring for 30 min, the solution was kept at 90 °C for 7 h in a water bath. Light yellow acicular single crystal (C6H5NH3)2Pb3I8·2H2O can be obtained after crystallization in two weeks, which was collected with vacuum filtered, washed several times with absolute ether, and dried in a vacuum oven for 12 h at 70 °C. The scanning electron microscope (SEM) image of the sample was examined using a JEOL JSM-6700F field-emission scanning electron microscope (FE-SEM), with an accelerating voltage of 10 kV. A suitable crystal with size of 0.220 × 0.210 × 0.180 mm was selected for single crystal X-ray diffraction analysis. Crystallographic data was collected on a Bruker Apex II CCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature. The structure was determined by the direct method using the SHELXTL-2014 program. To refine the structures, anisotropic thermal factors were employed for the non-H atoms. The hydrogen atoms of water in the crystal could not be found in the Fourier map because of the disordered arrangement. UV–Vis absorption spectra were recorded on a Shimadzu UV-2550 spectrometer. Fluorescence spectra were measured with a Hitachi F-4600 fluorescence spectrophotometer by a 280 nm excitation from a Xenon lamp as the excitation source at room temperature. The optical images were conducted on the fluorescent inverted microscope Nikon Eclipse TE2000U with 20 times magnification under UV lamp. Electron paramagnetic resonance (EPR) was carried out on Bruker EMX Plus at room temperature. The crystal structure of (C6H5NH3)2Pb3I8·2H2O was shown in Fig. 1, determined by single crystal X-ray diffraction. There are three Pb atom positions (Pb1, Pb2, Pb3) in one unit cell, with slightly distorted octahedral coordination. Each Pb atom is hexa-coordinated by iodine 2
Inorganic Chemistry Communications 109 (2019) 107562
M.-Y. Zhu, et al.
Fig. 2. (C6H5NH3)2Pb3I8·2H2O dispersed in DMF: (a) UV–Vis absorption spectra, inset is the absorbance v.s. the concentration ratio of Fe3+ over the sample, (b) fluorescence titration spectra, (c) the fluorescence intensity, inset is the corresponding calibration between the fluorescence intensity and Fe3+ concentration; Fluorescence results in blank and presence of various metal cations: (d) emission spectra, (e) fluorescence intensity, (f) selectivity.
Fig. S3. The sample shows bright fluorescence in Fe3+ blank (Fig. S3a), fluorescence quenching appears as the Fe3+ concentration is 1.0 × 10−5 mol/L, which is consistent with the result in Fig. 2(b). The fluorescence intensity at 339 nm was plotted as a function of the Fe3+ concentrations, as shown in Fig. 2(c), the inset illustrates Fe3+ concentration v.s. emission intensity, showing a linear relation. Quantitatively, the effect can be fitted as the equation If = 475.9–3.303 × 106 CFe(III), with a linear factor of R2 = 0.9965, meaning that Fe3+ has a strong interaction with the sample. The association constant (Ka) is 3.303 × 106, revealing an extremely strong quenching effect on the (C6H5NH3)2Pb3I8·2H2O sample [22,23]. The detection limit was measured to be as low as 1.0 × 10−7 mol/L through the titration method, which is superior to other methods regarding Fe3+ detection in solutions [24–26]. In addition, all the fluorescence data were recorded directly in the Fe3+ titration process, implying fast response, good for rapid detecting Fe3+ quantitatively. Based on the equation LOD = 3δ/s (where δ is the standard deviation of the signals and s is the slope of the linear calibration plot) [26,27], the limit of detection (LOD) was calculated to be 7.51 × 10−8 mol/L. The linear range and LOD of fluorescence sensing materials for Fe3+ detection were summarized in Table 1 [26–31]. The linear range and LOD of the developed sensing material (C6H5NH3)2Pb3I8·2H2O are improved over those of most reported fluorescence sensing materials in Table 1. It is worth noting that the
atoms. Carbon atoms in aniline vibrate at two symmetric positions, and water molecules situated between layers (Fig. 1a and b). Obviously, it is a 1D structure isolated by larger organic aniline. The conduction band (CB) and valence band (VB) of these perovskites are derived primarily from the inorganic lead iodine octahedron layer. Viewing along c-axis, layers are composed of alternate organic aniline groups and inorganic octahedra (Fig. 1c), which is favorable for electron-hole recombination between VB and CB. Infinite lead iodide octahedral chains are constituted by the Pb3I8 groups, surrounded by anilines (Fig. 1d), which might be a good structural characteristic for a fluorescence quenching sensor. The morphology of the obtained crystal is acicular, with a length of ~2 mm, width of ~30 μm, and thickness of ~8 μm (Fig. S2), consistent with the self-regulation principle in crystal growth. Crystallographic data has been deposited as CCDC 1864099, and crystal details and refinement results for (C6H5NH3)2Pb3I8·2H2O can also be found in the Supporting Information. Fluorescence quenching performance of (C6H5NH3)2Pb3I8·2H2O for detecting metal cation were investigated using a signal transduction pathway by titration in DMF. Since the as-synthesized sample is stable under sunlight, it was dispersed directly in DMF to form solution. Fig. 2 (a) illustrated the UV–Vis absorption spectra of the solution with different Fe3+ concentrations at room temperature. According to the UV–Vis absorption intensity of the solution, optimum excitation wavelength for the fluorescence measurement was chosen as 280 nm. At the excitation wavelength of 280 nm and a slit of 2.5 nm, the sample exhibits a broad fluorescence peak centered at 339 nm, as can be seen in Fig. 2(b). Fluorescence titration spectra were carried out in the presence of different Fe3+ concentrations ranging from 1.0 × 10−7 to 1.0 × 10−3 mol/L. The fluorescence intensity (If) decreases as the Fe3+ concentration (CFe(III)) increase from 1.0 × 10−7 to 1.0 × 10−3 mol/L, and the fluorescence is almost quenched as the Fe3+ concentration is 1.0 × 10−5 mol/L. In order to observe the fluorescence quenching effect intuitively, optical images of sample in DMF with Fe3+ concentrations ranging from 0 to 1.0 × 10−5 mol/L were recorded on a fluorescent inverted microscope with 20 times magnification under UV lamp, as presented in
Table 1 Comparison of fluorescence sensing materials for Fe3+ detection.
3
Sensing materials
Linear range (μmol·L−1)
LOD (μmol·L−1)
Ref.
FNCD N-CQDs [Zn2(oba)2(bpy)] Cd-MOF CuNCs AuNCs-PbS-QDs (C6H5NH3)2Pb3I8·2H2O
2–25 3.32–32.26 50–250 0–16 1–100 3–40 0.1–100
0.9 0.7462 0.3 0.3 0.3 1.5 0.0751
26 27 28 29 30 31 This work
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Fig. 3. EPR spectra of Fe3+ as absence and presence of (C6H5NH3)2Pb3I8·2H2O in DMF with the X-axis using g-factor (a) and magnetic field (b). Table 2 The standard electrode potential of different metal ions. Ion pairs
Mn2+/Mn
Ni2+/Ni
Cd2+/Cd
Ca2+/Ca
Mg2+/Mg
Zn2+/Zn
Cr3+/Cr
Co2+/Co
Al3+/Al
Pb2+/Pb
Fe2+/Fe
Hg2+/Hg
Fe3+/ Fe2+
E°/V
−1.030
−0.257
−0.403
−2.868
−2.372
−0.762
−0.744
−0.280
−1.662
−0.126
−0.447
0.790
0.770
lowest detection concentration for Fe3+ in this work is 0.1 μmol·L−1, which is even lower than the LOD of most reported materials [26–31]. Thinking about the interference of other metal cations, selectivity towards different metal cations was measured under same condition. As can be seen in Fig. 2(c), the fluorescence intensity was decreased by 97.41% with 4.0 × 10−4 mol/L Fe3+, so this concentration was chosen to investigate the selectivity of (C6H5NH3)2Pb3I8·2H2O with other 13 metal cations. The fluorescence emission spectra towards various metal cations are shown in Fig. 2(d). Influences on the fluorescence intensity at 339 nm are different with various metal cations, as illustrated in Fig. 2(e). Compared with other metal cations, Fe3+ displays obvious fluorescence quenching effect. The selectivity of sample to a certain metal cation (M) is defined as:
SM = (F0
hole recombination in the sample via capturing electrons. Standard electrode potential (E°) usually represents the ability of gaining electrons, the higher the electrode potentials, the higher the ability for getting electrons [40]. As listed in Table 2, standard electrode potential of Fe3+/Fe2+ and Hg2+/Hg+ cation pairs are high, so they exhibit stronger fluorescence quenching effect than other cations. Furthermore, ion diameter (D) might be another factor affecting the fluorescence quenching effect. The diameter of Fe3+ (D = 1.1 Ǻ) is smaller than that of Hg2+ (D = 2.0 Ǻ), thus fluorescence quenching effect of Fe3+ is higher than that of Hg2+ cation [40,41], as more charge with smaller ion diameter means stronger polarization and electron gaining potential. The CB and VB of these perovskites semiconductor are derived primarily from the inorganic lead iodine octahedra layer, so a proper ion size is needed to penetrate into the anionic chains for efficient electron transfer. The fluorescence quenching mechanism might be thought as the organic aniline layer inhibits metal ions with bigger diameter from approaching inorganic layer of lead iodine octahedra in the perovskite. On the basis of electron capture, Fe3+ shows the best fluorescence quenching effect via inhibition of radiative electron-hole recombination in the perovskite, which provides an effective method to detect Fe3+ quantitatively. In summary, a novel organic-inorganic hybrid perovskite (C6H5NH3)2Pb3I8·2H2O single crystal has been synthesized through a solution method. It has a unique perovskite structure that 1D lead iodide octahedron chains is constituted by the Pb3I8 groups, which is surrounded by anilines. Viewing from c-axis, layers are composed of alternate organic aniline groups and inorganic octahedra, which is favorable for radiative electron-hole recombination between the VB and the CB. This structure characteristic is good for fluorescence quenching sensor. The obtained organic-inorganic hybrid 1D perovskite (C6H5NH3)2Pb3I8·2H2O shows excellent fluorescence quenching performance as Fe3+ titration in DMF with a detection limit of 1.0 × 10−7 mol/L, including short reaction time, high sensitivity and selectivity. Based on the EPR results, the fluorescence quenching mechanism of (C6H5NH3)2Pb3I8·2H2O by Fe3+ can be attributed to the inhibition of radiative electron-hole recombination via capturing electrons. This work demonstrates the potential of these compounds in quantitatively detecting oxidizing metal cations.
F)/F0 × 100%
where F and F0 are the fluorescence intensity in metal cations presence and blank. Detailed selectivity of (C6H5NH3)2Pb3I8·2H2O towards other metal ions are shown in Fig. 2(f). The selectivity towards Fe3+ is much higher than other metal cations, with a SFe(III) value of 97.41%, showing very good selectivity [22,32,33]. Among the tested metal cations, Fe3+ exhibits obvious quenching effect on the sample. Since Fe3+ has five unpaired electrons in d orbital, electron paramagnetic resonance (EPR) was taken to investigate the fluorescence quenching mechanism. As shown in Fig. 3, the EPR spectra of Fe3+ are measured in blank and presence of (C6H5NH3)2Pb3I8·2H2O in DMF. The EPR signal at g = 2.16 in Fig. 3(a) can be ascribed to Fe3+ cation [34,35], and the signal observed in Fig. 3(b) can also be attributed to Fe3+ [36–38]. Furthermore, the concentration of the sample can be calculated qualitatively by integrating areas of detected peaks [35,39]. Comparing the EPR signal intensity of Fe3+, the intensity decreases obviously as sample presents. This implies that Fe3+ might react with (C6H5NH3)2Pb3I8·2H2O, leading to decreased Fe3+ concentration. Similar with light emission and absorption in MAPbI3, the electron-hole separation in (C6H5NH3)2Pb3I8·2H2O can generate photocurrent under sunlight radiation (Fig. S5), and the fluorescence is resulted from the radiative recombination of electrons and holes [5,8,12]. It is reasonable that Fe3+, as an oxidant, gain electrons from the sample, resulting in fluorescence quenching. Namely, Fe3+ inhibits the radiative electron4
Inorganic Chemistry Communications 109 (2019) 107562
M.-Y. Zhu, et al.
Acknowledgments
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